Insulators are used with electrical transmission and distribution systems to isolate and support electrical conductors above the ground for overhead power distribution and transmission. In power distribution systems, the most common insulator types are Pin-type and Post type insulators mounted on wood cross-arms to mechanically support the line conductors. These insulators are primarily designed for static loads but may be subject to dynamic loads, such as wind induced vibrating conductors or heavy objects falling on the line such as tree branches; therefore, they must withstand complex loads with compressive, cantilever, tensile and rotational force components. Pin-type insulators were developed in the nineteenth century and are still commonly applied to circuits today. As electrical networks and loads grew, with higher voltage systems and larger conductors, the Post type insulators were developed in the 1940's to better support these systems.
Traditional manufacturing of these Post type insulators is based on the wet-process porcelain process, also known as ceramics, by forming a body and cementing it to at least one ductile metal end-fitting. It is widely employed today to produce cost-effective insulators. Non-ceramic insulator manufacturing, also known as polymer or composite, was developed in the 1960's to overcome the high-weight and poor impact resistance characteristics of ceramics. The non-ceramic post insulators are comprised of metal end-fittings, a fiberglass core strength member and an outer weathershed, typically of elastomeric material. The fiberglass core provides mechanical strength sufficient to support high-voltage electrical conductors in both vertical and horizontal mounting configurations. Current manufacturing methods permanently attach the metal end-fittings to the core, most commonly by a mechanical compression method known as crimping or swaging.
FIG. 1 is an illustration of a fiberglass core 14 with metal end-fitting 12 used in a composite line post insulator 10, in accordance with the prior art. The metal end-fitting 12 of the composite line post insulator 10 may be crimped around the fiberglass core 14 with an eight-sided die compression machine, or a similar machine, which may result in imprints 16 within the metal end-fitting 12 where the die compression machine made contact. Other methods to secure the metal end-fittings to a fiberglass core may include an embedded substance, such as epoxy, and wedge or collet features to permanently fix the core in the end-fitting.
Non-ceramic insulators have grown in usage to be the preferred insulator for many post applications due to light weight, high strength, flexibility and resistance to impact; however, they have a high per unit cost when compared to conventional ceramic units. The current manufacturing methods require substantial machining, labor and equipment or materials to permanently attach the insulator components in the assembly process. The compression crimping method, though effective and preferred over other methods for cost advantages, is nonetheless costly, time-consuming and alters the surface of the fiberglass core.
The crimping process requires a machining operation to provide a precise known inside diameter on the end-fitting. This is performed by boring a precise known hole diameter to accept the fiberglass rod. After the machining operation, the flange portion left on the end-fitting about the hole has the desired wall thickness for the crimping operation. Another disadvantage of the crimping method is the creation of micro-fractures in the core material which can jeopardize the strength rating. Precision controlled crimping machines, equipped with acoustic monitors, can avoid excessive fracturing yet some amount is required to achieve a permanent attachment. Use of this method requires that the core member be sized to meet design load requirements after its outer layer is fractured. In other words, the core could be reduced in diameter if a non-compression attachment method was utilized.
Thus, a heretofore unaddressed need exists in the industry to address the aforementioned deficiencies and inadequacies.